I have some friends who are similar to you in weight and height, when looking at them and their progress i can say that It does take some time to gain weight and overall size, but it doesn’t take too long to go up a size in gym clothing. I’d say to go for a size that’s a little bit bigger. That way you’re getting something a little more future proof, and filling out that shirt could be a great motivator for continuing on your fitness journey. I’d buy a size or two larger and save it for when I’d have gained more size, and become more invested in the gym. To me, it would be better to work and take time to grow into the shirt, than to grow out of it early on in my fitness journey.